1. Field of the Invention
The present invention relates to a method and system for embedding information such as a copyright notice in data, and more particularly to a digital watermark embedded in an image.
2. Description of the Related Art
Digital watermarking is applied to various contents. A watermark embedded in an image will be described by way of example. Information of a digital watermark is embedded in an image by modifying pixel values such as luminance components, and frequency components so that a set of images represents particular information of xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d. In conventional techniques, for example, in xe2x80x9cStill Image Data Hiding in Pixel Blocksxe2x80x9d, Shimizu et al., Proc. of IPSJ 53rd Annual Conference, 1996, the values of two pixels are changed minutely to represent xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d based upon a difference of values between two pixels. However, the following two conditions are required to be satisfied, when a digital watermark is to be embedded.
1) It is necessary that an image embedded with watermark information hardly changes from an original image (that the watermark is invisible).
2) It is necessary that embedded information is hard to be erased during an image conversion process such as JPEG compression (xe2x80x9cDigital Compression and Coding of Continuous-Tone Still Imagesxe2x80x9d, ISO/IEC/0918-1).
An error rate of each bit subjected to JPEG compression is 0.1 to 0.2. In order to lower the bit error rate of JPEG compression to a xe2x88x928 power of 10, it is necessary to provide each bit of an embedded watermark with a redundancy of ten times or higher, so that the values of a number of pixels are necessary to be changed. Since the values of a number of pixels and frequencies are required to be changed in order to lower the bit error rate, there is a tradeoff between a reliability of watermark data detection and a quality of contents.
An image compression process such as MPEG is necessary for an image having a large amount of data such as a moving image. In such a case, it is essential to detect watermark data under the condition of MPEG compression, i.e., under the condition of MPEG streams. It is also necessary to prepare a fundamental function of detecting a watermark under the image condition. In this case, a detector system is required to have a MPEG decoder, increasing its cost and process time.
It is an object of the present invention to provide digital watermark embedding/detecting techniques capable of minimizing the number of pixels to be changed in order to embed a digital watermark, while a reliability of detecting embedded data is maintained high.
It is another object of the invention to provide a digital watermark embedding method and system capable of suppressing as much as possible the quality of contents from being degraded upon modification of pixels.
It is still another object of the present invention to provide a digital watermark detecting method and system capable of detecting embedded data not only under the image condition but also under the condition of compression streams.
In order to achieve the above objects, the invention provides the following four solution methods.
According to a first aspect of the invention, a first data embedding and detecting approach is provided in which data is embedded in a two-dimensional image as a position pattern of blocks whose pixel values are changed. Accordingly, a reliability same as that obtained through conventional techniques can be retained irrespective of that the number of pixels whose values are changed is smaller.
According to a second aspect of the invention, a second data embedding and detecting approach is provided in which a sum (or average) of pixel values of partial images is changed to a specific value. Accordingly, it becomes possible to optimize the change amount of pixel values of partial images.
According to a third aspect of the invention, a data embedding system is provided which uses the first and second data embedding approaches.
According to a fourth aspect of the invention, a data detecting system is provided which uses the first and second data detecting approaches.
According to a fifth aspect of the invention, an approach is provided in which the first and second aspects are combined, namely, an image is divided into blocks of kxc3x971 pixels and a sum or average of pixel values of each block is changed to a value satisfying particular conditions.
According to a sixth aspect of the invention, an approach is provided in which the size of each block embedded with watermark information based upon the fifth aspect is set to the size (e.g., 8xc3x978 pixels) of a block used as the processing unit of a compression process such as JPEG and MPEG. Accordingly, embedded data under the image condition and under the condition of compression streams has one-to-one correspondence so that the embedded data can be detected both under the image condition and under the condition of compression streams.